Electrical impedance tomography (EIT) is a non-invasive imaging technique used to investigate and measure regional lung ventilation and perfusion (flow of blood) in humans and animals. In contrast to conventional methods, EIT does not require the patient to breathe through a tube or sensor, does not apply ionizing X-rays and can be used for extended periods, say 24 hours or even longer. EIT can be used continuously and is therefore suited for monitoring treatment effects in real time and over time. EIT was first used to monitor respiratory function in 1983 and remains the only bedside method that allows continuous, non-invasive measurements of regional changes in lung volume, blood flow, and cardiac activity. More details of this technique can be found in “Electrical impedance tomography” by Costa E. L., Lima R. G., and Amato M. B. in Curr Opin Crit Care, February 2009, 15(1), p. 18-24.
In EIT, as disclosed by U.S. Pat. No. 5,626,146, a plurality of electrodes, typically 8 to 32, are arranged on the surface of the body to be examined. A control unit ensures that an electrical signal, for example a current is applied to one or several pairs of electrodes on the skin to establish an electrical field which in turn is measured by the other electrodes. The electrodes used to apply current are called “current injecting electrodes” although one of them might serve as reference ground. Typically, 3 to 10 mA RMS are injected at a frequency ranging from 0.1 to 10000 kHz. With the remaining electrodes, the resulting voltages are measured (forming the “EIT data vector” or the “scan frame”) and subsequently used to estimate the distribution of electric impedance within the body. Specific algorithms were developed to convert the set of voltages into images. These conversions are subject to two major challenges: the first is that the mathematical problem is ill-posed and non-linear, the second is inaccuracies of the measured voltages due to variations in amplifiers and current sources.
To overcome the ill-posed nature of impedance estimation, most EIT imaging algorithms make use of additional assumptions, restrictions or constrains. Typical methods known in the art are the use of a-priori knowledge about the internal structure of the medium and regularization to select a particular solution. Examples of a-priori knowledge include anatomical structures, functions of organs, physical characteristics of tissue like conductivity, blood flow, timing of heart contraction, and the like.
In the case of respiratory monitoring, a-priori knowledge can be derived, for example, from flow or volume measurements at the airway opening or from an X-ray image of the chest or from a CT scan, giving the contour and major structures of a patient's chest. Regularization methods enable to algorithmically decide between competing solutions, producing an image that is a reasonable estimation of the true impedance distribution within the thorax. Anatomical and physiological knowledge as well as physical laws form the basis for regularization methods which are known in the art. For example, abrupt changes in intra-thoracic impedance distribution are usually discarded as non-physiological. Gravity influences the distribution of blood pool and blood flow and therefore the distribution of impedance. Depending on the posture, the disease of the patient and the intra-thoracic location of the impedance distribution, gravity has significant effects on the measured signals. It is known that mechanically ventilated intensive care patients in supine position suffer from regional lung collapse in the dorsal regions of the lungs. Such collapse can lead to or aggravate acute lung injury. Postural change, for example turning the patient to the side or on his front (prone position) may reverse the collapse and can thus have beneficial therapeutic effects.
A three-zone-model may be used to demonstrate the influence of gravity (Hedenstierna G. et al. Pulmonary densities during anaesthesia. An experimental study on lung morphology and gas exchange. Eur Respir J. 1989 June; 2(6):528.) The three zones of this model are:
Zone 1: open and well aerated alveoli;
Zone 2: unstable alveoli in which their opening and closing occurs during the respiratory cycle;
Zone 3: collapsed alveoli.
These zones develop for example as a result of patients lying on their back (supine position) or on their stomach (prone position). In healthy subjects, the zones usually disappear in the upright position. The degree or level of zone expression within the lungs may vary with respect to the gravity vector. But the degree or level of zone expression usually remains unchanged on a horizontal plane orthogonal to the gravity vector. Ventilation-induced lung injury due to the cyclic opening and closing of lung units is assumed to happen mainly in zone 2. Hypoxemia is caused by the shunting of blood through the non-aerated zone 3. It is a treatment goal to eliminate those two zones in patients.
In mechanically ventilated patients, oxygenation can be improved by changing the body position of the patient. The mechanism behind such improvement is that collapsed lung spaces, described as Zone 3 above, are being opened in the new body position and thus oxygenation of blood is improved. Rotating the body of a mechanically ventilated patient into defined lateral positions to improve lung function is known in the art as disclosed in international application WO2005/094369.
Based on above knowledge, it might seem quite obvious to use EIT to monitor the operation of the lung to detect dysfunctions such as a collapse of the lung and the reversal of this collapse. However, in practice collapsed areas are difficult if not impossible to see on EIT images.
To overcome inaccuracies of measured voltages in the electrical impedance tomography method, it is known to use time-difference images, i.e. images that are calculated with reference to an image taken at a particular previous point in time. Such time-difference images are generated from changes in impedance relative to a baseline or reference condition. This relative or differential approach cancels out systematic measurement errors as well as some errors related to incorrect assumptions about thoracic shapes, body composition and contact impedance, since the same errors are assumed to be present in all images in a proportional way. Plotted rapidly in sequence, like a movie, these images create a representation of gas and blood flow in and out of each lung region and allow the care giver to evaluate lung function in real time. Thus, the dynamics of organ functions such as the beating of the heart and the breathing of the lungs can be monitored. Pre-requisite for stable time-difference images are a sound reference image. For this purpose, the sum or average of all values of a scan frame (composite EIT signal or plethysmogram) is often used. It is assumed, that the onset of a breath (start of inspiration) corresponds to a local minimum of the composite EIT signal and the reference image is therefore taken exactly at this point. However, in patients with small tidal volumes and low signal-to-noise ratio, for example in intensive care patients, the composite EIT signal is weak and exact determination of the onset of inhalation is nearly impossible. With the EIT instrumentation available at the present, artefacts create false signals very easily, introduce errors and bias, and ultimately lead to erroneous clinical decisions.
Document WO 2006/121469 A1 describes an EEG system comprising a cap with electrodes and motion sensors. The electrodes record the EEG signals. The motion sensors acquire motion data, which may include noise signals associated with the patient movement, blood flow motion and ballistocardiac motion within the patient. The data are processed to reduce motion noise from the EEG signals.
Some recently published EIT methods are the following:
WO 00/33733 A1 refers to a method for the regional determination of the alveolar opening and closing of the lung depending on the respiration pressure, wherein with the method of electrical impedance tomography an impedance signal is measured in at least one lung zone depending on the respiration pressure.
Document WO 2009/035965 A1 discloses an instrument and a method for assessing regional oxygen uptake by a patient. For achieving this, two electrical impedance tomography images measured at different times are compared. One image is taken shortly after inhaling the second image is taken after holding the breath for some time. Regional differences in lung volume are interpreted to correlate with oxygen consumption.
Document US 2004/034307 A1 is concerned with reflection tomography imaging using wave field energy such as ultrasound. This method is performed on a body which is immersed in a liquid filled container. Sensors and receivers are positioned at a distance from the body.
In the scientific paper by Brunner et al., titled “Imaging of local lung ventilation under different gravitational conditions with electrical impedance tomography”, in ACTA Astronautica, Pergamon Press, Elmsford, G B, Bol. 60 No. 4-7 (2007), it is referred to EIT imaging of lung ventilation under different gravitational conditions. Regional lung ventilation depends on the amount and direction of applied gravitational force. The gravitational influence varies with the position and orientation of a test person. The paper describes how parameters, which are attributed to four regions of interest, change depending on the tilting angle of a test person.
Above scientific paper demonstrates the influence of gravity on lung perfusion and lung ventilation. The existence of regional differences is known in the art and methods to expose these differences are disclosed by the cited publications and others. However, none of the hitherto disclosed methods allows for compensation of gravitational influences and artefacts.
A need for improved EIT instrumentation and analysis methods exists, which allow to monitor lung function and direct therapy. In particular long term EIT observation is expected to improve diagnosis and subsequent treatment. For example, due to continuous EIT monitoring, regional lung ventilation and regional lung collapse could be evaluated, the potential for lung injury assessed, and life saving treatment options, for example lung recruitment manoeuvres, initiated. Especially intensive care patients could greatly profit from an improved electrical impedance tomography technology and continuous monitoring by EIT.
Therefore, it is an advantage of the present invention to provide a device and a method that is able to measure and compute reliable EIT difference images. It is another advantage to provide a device and method that allows one to monitor lung function accurately and in real time. Furthermore, it is an advantage to create improved reference images and to improve utility and usability of EIT technology. Moreover, it is an advantage to improve regularization methods.